1. Field of the Invention
This invention relates to subframe mounts, and in particular a subframe mount capable of rate adjustment without removing the mount from the subframe.
2. Description of Related Art
Motor vehicles are generally constructed with a unibody frame that serves, among other things, to support the engine. Many motor vehicle designs today also include a subframe assembly attached to the unibody. The subframe assembly provides additional support to the engine, which may be mounted on unibody in the vicinity of the subframe. Generally, a mounting provision, usually in the form of a subframe mount, is disposed between the unibody and the subframe. These mounts typically provide a mechanical buffer between the unibody and the subframe.
A primary function of subframe mounts is to dampen vibrations between the subframe and the unibody. This damping is desirable to prevent vibrational damage to the subframe and to inhibit the transfer of vibrations to the passenger compartment. Vibrations may be caused by engine operation or uneven road surfaces. Typically, damping can be achieved by constructing the subframe mounts from an elastomeric material, which provides some degree of compliance to the mounts. The degree of compliance, or stiffness, of the subframe mount will govern what frequencies of vibrations the subframe mount can dampen.
Generally, the stiffness of the subframe mount is called the rate. During motor vehicle development, the subframe mount rate needs to be adjusted or tuned to optimize the damping effect. Typically this is performed by constructing many different subframe mounts having varying rates and testing those mounts on a vehicle until the optimal rate is achieved. Since testing typically occurs with an assembled motor vehicle, changing a mount with an undesirable rate typically requires removing the engine, removing the subframe from the unibody, removing the mount, replacing the mount with a new mount having a different rate, and replacing the engine. Each of these steps is a laborious process, leading to long development time for optimizing the subframe mount.
Adjustable rate subframe mounts are generally known in the art, typically to enable a motor vehicle to select a rate during operation of the vehicle. For example, U.S. Pat. No. 3,730,462 to Dick, teaches a tunable subframe mount assembly. The subframe mount assembly in the Dick design includes an inner cylindrical bushing and an outer cylindrical bushing. The outer cylindrical bushing is shaped like a cage, including a pair of axially spaced bars and a pair of axially extending bars positioned on opposite sides of the cylinder of the outer bushing. Concentric layers of metal cages having a similar shape as the outer bushing and separated by resilient bodies are disposed between the bushings, and the resilient material is positioned only between the axially extending portions.
For the Dick design, the subframe mount assembly is most stiff in the vicinity of the axially extending bars. The varying rates of the subframe mount are fixed within the mount, and stiffness tuning is available only by changing the orientation of the subframe mount. This design does not provide for modifying the rate of the subframe mount without modifying the orientation of the subframe mount. In other words, in order to change the rate, the subframe mount would have to be removed from the subframe and re-oriented.
In an alternate approach, Japanese Publication Number JP58106237 to Yoshida teaches a subframe mount having adjustable vibration damping characteristics. The subframe mount includes a metal frame containing a hollow shaft centrally disposed within the metal frame. An elastic member is disposed between the hollow shaft and the metal frame. Two hollow parts or voids are formed within the elastic member on opposite sides of the hollow shaft so that the spring constant of an axis passing through the hollow parts is reduced. A worm is engaged with a worm wheel that is fixedly attached to the metal frame. This worm gear is configured to rotate the metal frame so that the hollow parts may be positioned at different orientations so that the axis of reduced spring constant may be changed depending upon the driving conditions (for example, idling versus normal driving).
In the Yoshida design, the stiffness tuning is achieved by virtue of a worm gear rotating the mount, so that re-orienting the hollow parts with respect to the frame repositions the axis having the lowest spring constant. The selection of the orientation is altered during use of the mount.
Therefore, a need exists in the art for a subframe mount capable of having its rate adjusted without removing the mount from the motor vehicle to adjust the rate or re-orienting the mount.